This invention relates generally to current source inverters, also known as controlled current inverters, and more particularly to a method and means for limiting the voltage on the thyristors of a current source thyristor inverter.
In U.S. Pat. No. 4,230,979, entitled, "Controlled Current Inverter And Motor Control System", issued to Paul M. Espelage, et al. on Oct. 28, 1980, and assigned to the present assignee, there is disclosed a typical example of a current source inverter motor drive system including a thyristor type current source inverter.
The peak voltage on the thyristors of a current source inverter is proportional to the sum of the peak value of the sinusoidal voltage applied to the alternating current (AC) terminals of the inverter and the value of direct current (DC) commutated by the inverter. Prior art techniques utilized for limiting this voltage include power level clamps or surge suppressors to limit these voltages by force, or by the use of signal level methods to predict this voltage and thereafter limit current to a value which will not produce excessive voltage. U.S. Pat. No. 4,237,531, entitled, "Controlled Current Inverter System Having Semiconductor Overvoltage Protection", issuing to John H. Cutler and Loren H. Walker on Dec. 2, 1980, and assigned to the present assignee, describes not only a controlled current thyristor inverter, but also describes a predictive method and apparatus which transiently interrupts operation of the inverter when overvoltages across the thyristors are predicted. The inventive concept of U.S. Pat. No. 4,237,531 is applicable to thyristors of current source inverters whether used in motor drives as described in U.S. Pat. No. 4,230,979, or in var generators as described in the aforementioned U.S. Pat. No. 4,496,899. The only difference between motor drive and var controller applications is that in the case of the var generator, the thyristors which control the current are the same ones being protected. In an AC motor drive system, the inverter thyristors on the motor side of a DC link, coupling AC to DC source side converter and the motor side DC to AC inverter, are protected by changing the gating time; i.e., the firing angle of the thyristors in the converter connected to the source of AC power, due to the fact that the source side thyristors are the means by which current is controlled.